Ion exchange resins are normally solid materials which generally carry exchangeable ions. Due to their ability to exchange ions in a liquid without substantial alteration of the solid resin's structure, they are widely used in recovery processes such as the removal of undesirable components from water and aqueous process streams.
Generally, the most effective ion exchange resins are substantially insoluble but swellable to a limited degree in water or organic solvents and are resistant to physical deterioration such as excessive swelling or shattering. Moreover, in many applications, particularly when employed in continuous operations such as water treatment, the ion exchange resin is advantageously regenerated to prolong the lifetime of its utility and to reduce operation costs.
Conventionally, ion exchange resins are prepared by (1) haloalkylating a copolymer of (a) a monovinylidene aromatic such as styrene and (b) a cross-linking agent which is generally a polyvinylidene aromatic such as divinylbenzene in the presence of a Friedel-Crafts catalyst and (2) attaching ion active exchange groups to the resulting haloalkylated polymer. For example, an anion exchange resin may be prepared by aminating the haloalkylated polymer. See, Ion Exchange by F. Helfferich, published in 1962 by McGraw-Hill Book Company, New York; see also U.S. Pat. Nos. 2,694,702; 4,093,567; 4,900,796; 5,278,193 and European Patent No. 101 943. Unfortunately, these standard anion exchange resins contain large bead structures (300-1200 microns) and possess low regeneration efficiency.
During the water deionizing process, ion exchange resins attract and bind minerals and trace elements in the raw water. For example, cation resins exchange positively charged particles such as sodium, calcium, magnesium, and radium, whereas anion exchange resins attract negatively charged components such as carbonate, sulphate, chloride, nitrate, arsenate, arsenite, and chromate. After continuous use, the resin's capacity is gradually exhausted and it is eventually no longer capable of deionizing. At this stage, the resin must be regenerated. Both cationic and anionic resins can be regenerated, e.g., with a salt solution such as sodium chloride. In the case of cationic resins, the sodium ion displaces the cation from the exchange site; and in the case of anion resins, the chloride ion displaces the anion from the exchange site. The salts used for regeneration are released into the soil or sewer. In industrial scale water ion exchange (e.g. softening) plants, the effluent flow from the regeneration process can precipitate scale that can interfere with sewerage systems.
In the regeneration step, the concentration of solution needed to drive regeneration increases as the impurities penetrate the resin bead. Conventional resins contain ionic groups uniformly distributed throughout the resin and are often left with un-regenerated cores, causing leakage of impurities during subsequent purification steps. Such resins accordingly require long rinse periods and high concentrations of regenerant solution to achieve regeneration throughout the entire bead (including the core of the bead) to meet industry requirements for purification.
Methods for increasing regeneration efficiency are known in the art. Such methods include, e.g., optimizing base and or salt dose, base or salt flow rate, flow direction, and reserve setting. In particular, it is common practice to use excess base and/or salt and regenerant solution to optimize regeneration efficiency. Unfortunately, however, excess reagents for regeneration increase operating costs and waste discharge. Regeneration efficiency is also commonly optimized by using narrow grade resins (with a uniformity coefficient of less than 1.4) and/or resins having a small particle size (typically less than 500 microns). However these special grade resins have significant pressure drop limitations and have limited economic applicability.
In view of the deficiencies in the prior art methods, it would be highly desirable to provide new polymer resins containing anion exchange groups which provide more efficient ion exchange and regeneration. Furthermore, it would be desirable to provide a new efficient process for preparing these resins.